Jamie Condliffe

The placebo effect seems to make no sense: get mildly ill, take a dummy pill without knowing it's ineffective, and you'll recover in much the same way as someone taking real drugs. But new evidence suggests that we might have evolved the placebo effect to save energy.

New Scientist reports that new computer simulations seem to support an off-the-wall theory first put forward a decade ago: that sometimes for non-lethal illness, it pays for the immune system to not bother fighting. Because the immune system uses a lot of energy, so the theory goes, in days gone by it paid not to fight off infections if it could dangerously drain resources.

The model revealed that, in challenging environments, animals lived longer and sired more offspring if they endured infections without mounting an immune response. In more favorable environments, it was best for animals to mount an immune response and return to health as quickly as possible.

Basically, then, the human body might not have shaken off a millenia-old adaptation that helped our bodies fight off infection selectively, depending on our environment and resources. Now we're all well-fed and full of energy, though, it seems the placebo effect may just remain as a quirk of nature. [Evolution and Human Behavior via New Scientist]